They identified the specific device the order pertains to.
The reason the FBI suggested using the IMEI is that is one of the few ID's that can be looked up when a phone is locked. The other is the serial number printed on the back. They asked Verizon for it and Verizon looked it up. But the IMEI is on the SIM, not on the phone. Taking that SIM out and putting it in another phone means the FBI can run the SW on that new phone. Therefore the SW cannot be tied to the phone as requested in the order.